This invention relates in general to electrochemical cells and in particular to rechargeable lithium-ion cells. Such cells include a lithium intercalating anode, a lithium intercalating cathode and a lithium ion conducting salt dissolved in an organic solvent or mixed solvent solution.
An electrolyte additive was discovered which when added to lithium ion organic electrolytes was found to provide thermal stability to the electrolyte. Lithium ion conducting electrolytes generally contain lithium salts such as lithium hexaflurophosphate (LiPF6), lithium hexafluroarsenate (LiAsF6), and lithium perchlorate (LiClO4) which, when dissolved into organic solvents such as esters, cyclic esters, and dialkyl-carbonates, form Lewis acid solutions. In addition, trace water is always present in these solvents. The water can hydrolyze the ester solvents in the presence of Lewis acids into organic acids and alcohols resulting in rapid electrolyte decomposition and degradation of cell performance when used in lithium ion cells. The catalyst for this hydrolysis is the acidity of the organic electrolyte, and the rate of reaction is accelerated at elevated temperatures. Lithium ion cells are expected to be used over wide temperature ranges from +40xc2x0 C. Go 60xc2x0 C. and require stable storage to temperatures as high as 90xc2x0 C. Highly conductive organic electrolyte solutions containing for instance LiPF6 are known to rapidly decompose from hydrolysis at about 70xc2x0 C. which limits its practical use in cells. If the Lewis acid solutions such as those containing LiPF6 could be stabilized from thermal decomposition, the solutions would be better suited for use in practical lithium ion batteries.
The general object of the invention is to provide a salt additive to lithium conducting organic electrolytes which thermally stabilizes the solution and more particular to provide improved cell operation and storage of lithium ion cells at elevated temperatures.
It has now been found that these objects outlined above can be met by using LiBF4 as an additive to lithium conducting organic electrolytes in a salt molar ratio of at least 1:1. In a preferred embodiment, it was found that additions of lithium tetrafluroborate (LiBF4) in at least a 1 to 1 molar ratio to LiPF6 in a mixed solvent solution containing ethylene carbonate, propylene carbonate, dimethyl carbonate, and methyl formate in a 1:1:3:15 volume ratio, respectively, resulted in a solution which is thermally stable at a temperature of 71xc2x0 C. over those solutions containing less LiBF4 or only LiPF6.